A touch panel system is a data input device that allows an operator to interact with information rendered on a display screen. For example, the operator can select one of multiple computer command options rendered at different locations on the display screen by touching the screen at one of the locations. A touch panel system employs a position measurement apparatus that generates an address signal that is indicative of the touched location. The address signal is delivered to a computer that determines from the address signal which one of the command options is selected. The object with which the operator touches the display screen is called a stylus and may include, for example, the operator's finger, a pen, or a pencil.
A touch panel system of the capacitive-type typically includes a faceplate that has on its outer major surface an optically transparent, electrically conductive coating of a preselected resistivity. The faceplate is positioned in front of the display screen of a display device so that an operator can touch the conductive coating at locations aligned with information rendered on the display screen. The operator touches the conductive coating with a stylus having a nonzero, finite capacitance with reference to electrical ground. Such a touch panel system distinguishes the location the stylus contacts from the other locations on the faceplate by determining the location at which there is the capacitance characteristic of the stylus.
One type of capacitive touch panel system is described in Panttaja, "Touch screens let your fingers provide a fast, simple entry into the computer," Electronics, April 19, 1984, 140-144. The electrically conductive coating on the faceplate of the Panttaja system is patterned in the form of multiple electrically isolated, rectangular areas or "pads" positioned at different, fixed locations on the faceplate. Since each of the pads is electrically isolated, the Panttaja system is capable of supporting only a limited number of pads (i.e. up to about 32) to allow sufficient surface area on the faceplate for an electrical conductor connecting each pad to the touch detection apparatus.
Information to be selected by an operator is rendered on the display screen in alignment with preselected ones of the pads. The touch panel system employs the touch detection apparatus to detect contact between a stylus and any one of the pads. The touch detection apparatus generates a two-state output signal (i.e., TOUCH or NO-TOUCH) for each one of the pads and cannot distinguish between different locations on a single pad. Such a touch panel system suffers, therefore, from the disadvantages of being inflexible because of the fixed locations of the pads on the faceplate and being impractical for use in sophisticated applications because of the limited number of distinguishable touch locations.
U.S. Pat. No. 4,476,463 of Ng et al., describes a capacitive touch panel system having a rectangular faceplate with an electrically conductive coating. The conductive coating has a preselected resistivity, covers the entire outer major surface of the faceplate, and carries four bar electrodes. A different one of the bar electrodes extends along almost the entire length and near each of the side margins of the outer major surface of the faceplate. The bar electrodes form two pairs of opposed electrical contacts that define two orthogonal axes across the faceplate. Each bar electrode is electrically connected to the conductive coating and a touch locating circuit. One of the bar electrodes is also electrically connected to a touch detection circuit of the type employed in the Panttaja system. The touch detection circuit and the touch locating circuit cooperate to determine the location at which the stylus touches the faceplate.
The touch detection circuit first detects contact between the conductive coating and a capacitive stylus. Whenever the touch detection circuit detects such contact, the touch locating circuit then measures changes in the impedance of the conductive coating caused by the contact. The impedance measurement typically is performed sequentially with reference to each of the bar electrodes A microprocessor analyzes the impedance measurements obtained from the four electrodes to determine the location at which the stylus touches the faceplate.
In one embodiment, the touch locating circuit includes an impedance measurement signal source that applies a variable-frequency measurement signal of the square-wave type to successive ones of the bar electrodes, thereby to identify with respect to each electrode the location at which the stylus touches the faceplate. The square-wave signal switches between a first positive signal voltage and electrical ground. The signal source changes the frequency of the measurement signal applied to a bar electrode until the combined resistance-capacitance characteristics of the faceplate and the contacting stylus have a predetermined effect on the signal, as described below with reference to one of the four bar electrodes.
The measurement signal applied to the bar electrode is initially of a first frequency that is sufficiently low that the measurement signal is capable of charging and discharging the faceplate to the first signal voltage and ground, respectively. Such charging and discharging occurs despite the presence of stored charge resulting from the capacitive effects of the faceplate and the stylus. A comparator receives the measurement signal present on the faceplate and a positive DC reference voltage of lesser magnitude than that of the first signal voltage. Since the faceplate is completely charged and discharged, the comparator generates an alternating output signal. In response to such an alternating output signal, the measurement signal source incrementally increases the frequency of the measurement signal.
The measurement signal frequency incrementally increases to a sufficiently high level so that the faceplate voltage never drops below the reference voltage. As a result, the measurement signal delivered to the comparator maintains a voltage magnitude greater than that of the DC reference voltage, and the comparator generates an output signal of a substantially constant voltage.
The measurement signal frequency at which the comparator generates the output signal of constant voltage represents the location at which the stylus touches the faceplate relative to the one bar electrode. This frequency is compared with a reference frequency to identify the location. The reference frequency is the frequency at which the measurement signal causes the comparator to generate a DC output signal when no stylus is in contact with the faceplate and a reference capacitor is electrically connected between the opposed bar electrode and electrical ground. Information relating to the reference frequency is stored in a random-access memory and is compared by a microprocessor with the frequency relating to a touch location.
The above-described measurement is performed successively for each one of the four electrodes. The microprocessor then analyzes the four measurements to identify the touch location.
The touch panel system of Ng et al. suffers, however, from at least three disadvantages. First, the touch locating circuit identifies a touch position relatively slowly because the incremental frequency change of the measurement signal requires multiple applications of the signal to the faceplate. Second, the touch panel system is of a relatively complex design because the system employs both a touch detection circuit and a touch locating circuit. The design is further complicated in that the touch locating circuit employs two comparisons that include a comparison of the measurement signal frequency with a DC reference voltage to form an output signal and a comparison of the resulting output signal with a corresponding output signal for a reference frequency. Third, the relatively large area of the faceplate-covering conductive coating causes it to conduct stray electric fields. As a consequence, the touch panel system is susceptible to receiving electrical noise signals that can be of magnitudes greater than the magnitude of the measurement signal.